Radio Considerations for AX.25 Packet Stations

Summary

A high-level summary is presented here in quick, bulleted form. More details are provided below. It is important to understand that the recommendations for antennas really apply to any simplex FM station, whether it is used for voice or data communications.

Use a 25-50 watt "mobile" or "base station" radio whenever possible, even if you are near the BBS. Remember, it's not about the BBS hearing you. It's about you being able to hear all of other operators on the frequency and them being able to hear you.

Use a radio with a "data" connection so your TNC gets a consistent audio level and so you can monitor traffic with the speaker

Details

Transmitter Power

Generally speaking, use at least 25-50 Watts whenever possible, even when located very near to the BBS.

Remember: To avoid the hidden node problem, all users on the frequency need to hear all other users. So use as much power as necessary to ensure that other users on the channel can hear you.

Fixed sites are encouraged to use 50 Watts

Field sites are encouraged to use at least 25 Watts

Data Connector

The audio and PTT signals between the TNC and the radio can be connected via either the microphone and speaker jacks or a separate "data" jack designed for that purpose.

Not all radios have a data jack.

The "data" jack is preferred because:

It delivers a consistent audio level to the TNC, regardless of where the volume control is set

It allows monitoring of the packet activity with the speaker

It allows plugging in a microphone in case the radio needs to occasionally be used on voice nets

HT Radios

Hand-held radios or "HT"s are a poor choice for a radio for packet networking.

Hand-held radios are usually limited to 5 watts and some have even less power. That's just not enough power to be heard across several cities unless your antenna is up very, very high.

An HT should be considered a "last resort" in an emergency because its low power will cause the "hidden node problem" which will slow down the channel for everyone.

If an HT is the only radio available, then it's better than nothing. But be sure to hook it up to a recommended antenna, if available. Other packet users in surrounding cities will never hear you with just the "rubber duck" antenna.

Be sure to turn off the battery save feature. The battery save feature typically puts the radio to sleep for some fraction of each second. This will cause the radio to miss the beginning of most packets, causing lots of retransmissions to occur, and slowing down the channel for everyone.

Notes on Some Specific Radio Configurations

The SCCo BBSs use mostly Kenwood TM-V71 radios for 144 and 440 MHz. The data connector on the back is a 6-pin mini-DIN. Of course, only one side of the radio is used. But we have had to resort to using the other side when the receiver filters failed on one side. And the dual-band nature of the radio means that one radio can be a spare for either band.

The SCCo BBSs, the SCCo EOC, and the communications trailers use the Alinco DR-235 radio for 220 MHz. The data connector on the back is a DB-9. These radios are also used at the SCCo EOC and are in the SCCo ARES/RACES communications trailer and the SCCo Interoperability trailer used by County Comms.

The Kenwood TM-D710 is in use with an external TNC at a couple of SCCo BBS locations. The integrated TNC was used for a while at the BBS sites and works well, except that it can not be operated with open squelch. Operating with open squelch is important for a remote operation where the noise floor may change. So the radios are now used with external TNCs. But open squelch is not very important for a field station where the user is present to adjust the squelch. The integrated TNC does not have a mailbox function and lacks some of the added capabilities of full-function, external TNCs. But otherwise, it works very well at both 1200 and 9600 baud.

Many city EOCs and packet committee members use various models of Yaesu mobile radios for packet on 144 MHz. Models with a data connector on the back are preferred.